"The thing about this outbreak that is very perplexing in being able to track and get our arms around is the fact that about 40 to 50% of the infections are asymptomatic," says Fauci. "If that's not bad enough, we know that now—and there was some doubt about this until it was really clarified—it is likely that up to about 50% of transmissions off of individuals who do not have symptoms."